Navigational system for airways traffic control



March 11, 1952 o. s. FIELD ETAL 2,588,916

NAVIGATIONAL SYSTEM FOR AIRWAYS TRAFFIC CONTROL Filed Feb. 2, 1948 9 Sheets-Sheet l FIG-.1.

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s a! i INVENTORS 5 2 g og fi'eld andJEFr-eehafer 2 E r I E E E 7Z 1/M W Their ATTORNEY March 11, 1952 o. s. FIELD ET AL 2,588,916 NAVIGATIONAL SYSTEM FOR AIRWAYS TRAFFIC CONTROL Filed Feb. 2, 1948 9 Sheets-Sheet 8 F ROM GATE CIRCUIT 86 TO PLATE OF PHA ASTRON 0F BLOCK 1Z2 EN T 0R5 0556M cu zd J. EFreehaFer BYWMM Their ATTORNEY Fla],

Patented Mar. 11, 1952 NAVIGATIONAL SYSTEM FOR AIRWAYS TRAFFIC CONTROL Oscar S. Field and John E. Freehafer, Rochester, Y., assignors to General Railway Signal Company, Rochester, N. Y.

Application February 2, 1948, Serial No. 5,766

20 Claims.

This invention relates to navigational systems and more particularly to such systems adapted for use with an airway trafllc control system such as, for instance, is disclosed in the co-pending application of H. C. Kendall and L. H. Orpin, Serial No. 743,046, filed May 22, 1947, and assigned to the same assignee.

In this prior application is disclosed an airway traffic control system utilizing a series of timed synch pulses, each separated by a plurality of timed altitude or information pulses. These pulses are transmitted simultaneously by a plurality of ground stations arranged serially to form an air lane, and aircraft flying along this air lane contain receiving apparatus for receiving these pulses and transmitting apparatus for transmitting reply pulses to certain of them. Each aircraft apparatus includes gating circuits such that only pulses from the nearest ground station are effective to cause the aircraft to transmit reply pulses. Each ground station also includes gating circuits so timed that only reply pulses arriving from aircraft within a predetermined area surrounding the ground station are received by the ground station receiving apparatus. In order to allow each ground station to transmit to and receive information from a plurality of aircraft within its block boundary so defined, successive altitude or information pulses following each synch pulse are characterized for different altitude zones, and an altitudesensitive device on each aircraft controls its gating circuits such that only the altitude or information pulses intended for its particular altitude zone are effective at the aircraft. Provision is made at both the ground station and the aircraft for changing the characteristics of the pulses transmitted from each and communication links are also provided between ground stations so that an integrated system of communication is available which may, as described in the above application, be utilized to transmit to aircraft information as to adjacent traffic conditions. Visual indications in this prior application are disclosed in the form of indicating lamp signals in the aircraft cockpit, green for a clear lane ahead, red for occupancy by another aircraft ahead, etc., these lamps being controlled by the aircraft receiving apparatus above-mentioned.

Accordingly, the principal object of this invention is to provide a navigational system for use with an airway traffic control system which will provide a visual picture or map of the air lane and the position of aircraft relative to the vair lane.

Another object of this invention is to provide such a navigational system utilizing pulse transmission which may be synchronized with the pulse transmission of a cooperating airway traffic control system.

Still another object of this invention is to provide an indicator for such a system which will accurately show the length and/or azimuth of the legs of an air lane relative to a ground station, and the range and azimuth of aircraft from the same station as well as their heading relative to the airway.

Another object of this invention is to provide such a system which operates upon a fixed frequency so that no adjustment need be made to airborne apparatus from the time an aircraft leaves an airport until it arrives at its destination.

A further object of this invention is to determine at a ground station slant range to an aircraft, compute the horizontal range therefrom, and thereafter transmit this information to the aircraft.

A further object of this invention is to transmit information to an aircraft for conversion into a navigational system indication on a cathode ray tube my means of pulses of varying durations or paired pulses timed after an initial or sync pulse.

A further object of this invention is to provide such a system wherein the transmission is produced simultaneously with the transmission of pulses by a cooperating airway traffic control system transmitter but on a different frequency.

A still further object of this invention is to provide a means for accurately determining the center of a beam of radio energy arriving at an aircraft without the use of loop antennas such as, for instance, are utilized in goniometers or direction finders.

Other and further objects will appear during the course of the following description when taken with the accompanying drawings, in which:

Fig. 1 shows a display according to the invention as seen on the cathode ray tube indicator;

Fig. 2a shows portions of a wave form diagram illustrating the operation of the system;

Fig. 2b is an expanded view of a portion of the wave form diagram of Fig. 211;

Fig. 2c is a further expanded view of a portion of the wave form diagram of Fig. 2b.

Fig. 3 is a block diagram of the ground station components of the system;

Figs. la-4b are a block diagram of the airborne components of the system; I

Figs. 5a-5c show one form of an electrome chanical timer which may be utilized in the airborne equipment shown diagrammatically in Figs. 40-41);

Fig. 6 shows one form of pulse counting circuit which may be utilized in the airborne equipment shown diagrammatically in Figs. ia-4b;

Fig. 7 shows one form of range tracking circuit which may be utilized in the airborne equipment shown diagrammatically in Figs. la-4b;

Fig. 8 shows the indicator of Fig. 1 modified to produce more than two horizontal parallel lanes; and,

Fig. 9 shows one form of a lane switching circuit which may be used with the airborne equipment shown diagrammatically in Figs. 411-412 to indicate to a ground station position of the aircraft in the proper laneof more than two horizontal parallel lanes.

The system of the above application shows a means for dividing into a plurality of vertical altitude zones a section of an air route defined by ground stations arranged serially. Through the use of the navigational system described herein, each vertical altitude zone may be further divided horizontally into a plurality of lanes which preferably are parallel.

Brieny, this navigational system comprises the transmission of pulses from a rotating antenna and an omni-directional antenna at each ground station, the transmission of the pulses being synchronized with the transmission of pulses by the ATC (airway traffic control) system but on a different frequency. The explanation which follows will be given, in general, interms of pulse width for the sake of simplicity, but it is to be understood that in each case the use of paired pulses is also contemplated where the leading edge of the initial pulse of the pair is timed in the same fashion as the leading edge of the pulse of varying width exemplified, and the second pulse of the pair is timed after the first in the same fashion that the width of the explained pulse is varied and for the same purpose. Also, by secondary leg of an air lane is meant the leg which the aircraft traverses going toward a ground station within a block, and by the primary leg is meant the leg which the aircraft will traverse going away from the ground station toward the next ground station of the defined air lane. For purposes of explanation, it will be assumed that the ATC system is transmitting in the L-band of frequencies, and that the pulses transmitted by the navigational system are S-band pulses.

Initially, an omni-directional S-band pulse of distinctive width is transmitted from the navigational transmitter simultaneously with one of the L-band pulses (either sync or altitude) from the ATC transmitter. This pulse, when received at the aircraft, will be utilized to start the timin circuits of its navigational system and to produce a secondary leg marker on a cathode ray tube indicator. Two subsequent omni-directional S- band pulses, each of a distinctive width different from that denoting the secondary leg, are thereafter transmitted at times which are predetermined at each ground station to designate north relative to the azimuth of the secondary leg of the air lane and the azimuth of the primary leg relative to the secondary, respectively. The aircrafts apparatus includes a timer for each distinctive width other than that denoting the secondary leg, each timer being started by the first omni-directional pulse above-described (the secondary leg pulse) and stopped by the subsequent sponsive.

of the cathode ray tube.

omni-directional pulse of the distinctive width to which it is responsive. Each timer produces an output in the form of an angular displacement whose magnitude is proportional to the time between the two received pulses to which it is re- The angular displacement output of each timer is connected to a corresponding sineco-sine resolver whose output in turn is connected to the deflecting elements of a cathode ray tube. Thus when sweep voltages determined by the resolver in known manner are applied to the oathode ray tube, radial traces will be produced on the cathode ray tube screen at angular positions corresponding to the angular displacement of each timer.

In addition, an S-band pulse of still a different distinctive width is transmitted by a rotating directional antenna at each ground station simultaneously with each of the L-band pulses (either sync or altitude) from the ATC transmitter whenever an omni-directional S-band pulse is not being transmitted. Only c-rrtain of these directional pulses will be received at the aircraft, namely, those which are transmitted by the ground station during the time the directional antenna thereof is pointing at the aircraft. Equipment is provided in the aircraft for measuring the time between the r-' ception at the aircraft of each omni-directional S-band ulize ccrresponding to the secondary leg and these subsequently received directional S-band pulses, and this is fed through still another timer and associated resolver to the cathode ray tube to roduce on the cathode ray tube screen an indication of the azimuth of the aircraft relative to t-e secondary leg.

One additional bit of information is nece sary at the aircraft, namely, the range of it from the ground station. This is provided by transmitting a pair of omni-directional S-band pulses of still another different distinctive width from the ground station, the first of these being transmitted substantially simultaneously with the L- band pulse from the ATC transmitter correswnding to the altitude zone in which the aircraft is located. The second pulse of the pair is spaced after the first by a distance equal to the ground range of the aircraft relative to the ground station, as computed by the ground station. This is possible because the slant range to eachaircraft is available at each ground station due to the fact that the latter sends out L-band pulses and receives reply pulses thereto from each aircraft,

the time-elapsing between such transmission and reception being a measure of the slant range to the corresponding aircraft, as is too well known to require further explanation. Equipment is provided on the aircraft to measure the time elapsing between the two pulses of thispulse pair and apply an output proportional to this time to the cathode ray tube so as to produce an indication denoting the aircrafts range from the ground-station at the angular position of the aircraft with respect to the secondary leg which has been determined and applied to the cathode ray tube in the manner above-described.

The form which these cathode ray tube indications may take is illustrated in Fig. 1, which shows a view of the face or screen of a cathode ray tube indicator i. In this representation the nearest ground station, associated with the primary and secondary legs of the air lane shown, is to be understood as located at the center C The downwardly extending radial trace 2 indicates the secondary leg of the air lane.

Displaced from this at an angle (reading clockwise) is a dotted radial line 3 representing north with respect to the secondary leg 2. This line is shown dotted because it may or may not be shown on the cathode ray tube screen as desired. The primary leg of the air lane is indicated by radial line 4, making an angle with secondary leg 2. ,The aircraft itself is represented by dot 5, which as shown is at an angle a (reading clockwise) with respect to the secondary leg 2 and at a range r from the ground station at C. It is to be noted that the angles 0 and are fixed values for each ground station once the air lane has been determined and laid out, whereas the angle a is variable and depends upon the position of each aircraft with respect to the ground station at C. Since the direction of North is always known at the aircraft by means of its fiuxgate compass, this map on the face of the cathode ray tube indicator 1 can be oriented with respect to north in known manner by means of devices such as synchros, which may, for instance, be connected to the gear 6 of Fig. 1. Gear 6 engages with peripheral teeth on ring 1 on which are provided degree marks as shown, the synchro mechanism maintaining ring 1 so that the zero degree mark is always lined up with the north direction of the cathode ray tube indicator picture, which is indicated by dotted line- 3 as noted above.

The length of radial lines 2 and 4, representing the secondary and primary legs of the air lane respectively, may be made the same and controlled at the airborne apparatus, preferably being made long enough to extend almost the periphery of the face of the cathode ray tube I. However, in many cases it may be desirable to indicate at the aircraft the actual lengths of the secondary and primary legs of the particular block within which the aircraft is located. Usually, the secondary and primary legs will be of the same length, but, a more general situation will be assumed for purposes of explanation and exemplification in which the lengths of the secondary and primary legs differ from one another. Thus, as illustrated in Fig. 1, radial trace 4, representing the primary leg, is longer than radial trace 2, representing the secondary leg. The length of each leg of the air lane is, of course, fixed at the time the air lane is laid out. Information can, therefore, be transmitted from each ground station as to the length of each of its legs by means of a pulse of another distinctive width transmitted at the proper time after the initial pulse, which will hereinafter be denoted as of width III, utilizing the period of time between transmission of successive pulses of width III to denote the maximum length of an air lane leg which will ever be encountered along an air lane. These subsequent pulses, denoting the lengths of the primary and secondary leg, or the single pulse denoting both their lengths if they are of the same length, will also be transmitted simultaneously with the nearest L-band ATC pulse, either sync or altitude. Thus the length of each leg will be determined within an accuracy equal to the maximum length of air lane leg divided by twice the number of L-band pulses transmitted by the ATC system. As will be pointed out hereinafter, the exemplified ATC system transmits 1080 pulses per second, and hence the accuracy of the exemplified system is equal to the maximum length of air lane leg divided by 2160.

The aircraft operator would also like to be able to determine at a glance the heading, of his aircraft with respect to north. This may also be applied to the face of the cathode ray tube indicator I as line 8, the direction of this being obtained from the planes fluxgate compass via another synchro mechanism. In Fig. 1, the aircraft at 5 has a heading of 330 as indicated by line 8, whereas if it were flying properly parallel to the secondary leg 2 it would have a heading of approximately 20. To assist the aircraft operator to determine at a glance the amount of correction necessary to regain his correct course, an outer ring 9 may be provided as shown relative to the cathode ray tube 1 and having plus and minus degree marks as indicated. This outer ring 9 is made manually rotatable but may be held fixed at any desired angular position, and preferably is rotated as necessary so that the air lane leg being traversed (either secondary or primary) always points toward 0. Since as shown, the aircraft at 5 is traversing the secondary leg of the air lane, ring 9 is now aligned so that radial trace 2 points toward 0. Thus, by following a line from the center C through heading marker 8 and extending it out either visually or by means of an overlay, the aircraft operator can immediately see that a correction of 50 must be applied in order to regain his proper course under no wind conditions.

The ATC system disclosed in the above application transmits L-band pulses at a frequency of 1080 cycles per second, or in other words, one pulse every 926 microseconds. These transmitted ATC L-band pulses are in the form of a series of distinctively characterized sync pulses, each separated by 1'7 altitude pulses, which in turn each characterize a different altitude zone from 1 to 17. For convenience, the

transmission of a sync pulse followed by 17 altitude pulses is referred to as one sequence scan. Each sequence scan, or cycle, covers of a second, and 60 sync pulses are transmitted per second.

It is to be understood, of course, that an airway may be divided into altitude zones and each altitude zone maybe further subdivided into lanes. It is clear that in this generalized system it is possible to assign in an orderly manner to each altitude and each lane a definite interrogation pulse in a sequence scan to which an aircraft within the airspace defined by that altitude zone and that lane responds for the purpose of registering occupancy in the proper air space at the ground station and to which the aircraft equipment listens in order to obtain instructions addressed to it from the ground. Thus in the exemplified system in which seventeen altitude pulses constitute a sequence scan, it is to be understood that sufilcient channels are available to gather occupancy information and to provide signals for an airway comprising either one lane and seventeen altitudes or two lanes and four altitudes or four lanes and four altitudes or eight lanes and two altitudes or seventeen lanes and one altitude. The term altitude zone is used in a general sense to mean a block of air space defined by a lane and an altitude zone.

The present invention provides among other facilities a means for operating a lane switch whose function is to control the time at which the aircraft responds to the signals from the cooperating ATC' system in accordance with the lane which it occupies in the same way and for the same purpose for which the altitude switch controls the time of operation in accordance with the altitude occupied by the aircraft.

In Fig. 2a, line A represents portions of a series of such sequence scans, sync pulses being indicated by the letter S and separated from one another by 17 altitude pulses of which only the fourth is marked, since it will be assumed for purposes of the following explanation that only one aircraft is within the block boundaries of the ground station and that aircraft is within the fourth altitude zone. Line B represents the omni-directional S-band pulses sent up by a navigational system according to this invention, and line C represents similarly the directional S-band pulses transmitted by such a navigational system.

Referring to line B, it will be seen that initially an omni-directional pulse of width III is sent up coincident with the transmission of one of the L-band ATC pulses. It makes no difference whether this coincident L-band pulse is a sync or altitude pulse, and in this example the initial S-band pulse of width III is shown coincident with a pulse corresponding to the first altitude zone. Since as above noted, the aircraft is assumed to be in altitude zone 4, an omni-directional S-band pulse of width V is next sent up substantially coincident with the first L-band pulse corresponding to altitude zone 4 occurring thereafter, and followed by a second pulse of width V before another L-band pulse is transmitted. The time interval between the two pulses of width V denotes the projected range on the ground, or horizontal range, from the ground station to the aircraft as above-mentioned, and the manner in which this is determined will be pointed out more fully hereinafter.

Since the second pulse of a pair of range pulses can not be transmitted simultaneously with an L-band pulse, provision must be made to avoid interferences which arise from this circumstance. Accordingly, the stations along the airway are arranged to transmit range information in se-,

quence. Thus, for example, stations may be counted off along the airway in cycles of four. Stations number one transmit range information during the first sequence scan only; stations numher two transmit range information during the second sequence scan only, etc. Thus, stations which transmit simultaneously are separated by four blocks and presumably are not within interference distance of each other. The fact that the first pulse of a range pair is transmitted simultaneously with an L-band pulse makes it possible for the airborne equipment to recognize the sequence scan during which the closest station is transmitting range information and causes it to accept the second pulse of the pair. Thus, in the exemplified system of 17 interrogation pulses per sequence scan, range information reaches an aircraft fifteen times a second.

As noted above, each cycle represented in line A is /60 of a second, and an omni-directional pulse of width III is transmitted every 60 cycles, or one per second. Therefore, using the time interval between pulses of width III to mark 360, if a pulse were to be transmitted simultaneously with each L-band pulse occurring between the transmission of each pulse of width III, each such pulse would mark one third of a degree of angular rotation (360+l080). At the proper .time after the pulse of width III to determine the angle 0 (Fig.1) between the secondary leg bearings.

and north, an omni-directional S-band pulse of width II is transmitted, again simultaneously with the nearest L-band pulse. The angle 6 is thus transmitted accurately within one-sixth of a degree.

The next omni-directional pulse transmitted is a pulse of width V, the first of another pair denoting the ground or horizontal range to the aircraft, and substantially simultaneously with the next L-band pulse corresponding to altitude zone 4. This is followed by another pulse of width V, the time interval between the two pulses of width V denoting the horizontal range between the ground station and the aircraft as above-described. Thereafter, at a time determined by the angle (Fig. 1) an omni-directional S-band pulse of width IV is transmitted, again simultaneously with the nearest one of the L- band ATC pulses (either sync or altitude, as the case may be), and thus fixing the angle accurately also within one-sixth of a degree.

Omni-directional S-band pulses of width VI and VII are transmitted at times determined by thelengths of the secondary and primary legs respectively as above-described, and again simultaneously with the nearest one of the L-band ATC pulses as also illustrated in line B.

LineC represents the S-band pulses of width I which are transmitted from the rotating directional antenna of the ground station simultaneously with each L-band pulse whenever a pulse of width II, III, IV, VI, or VII is not being transmitted. This rotating directional antenna is driven at exactly 60 R. P. M. (1 R. P. S.) and its movement is synchronized so that it is pointing exactly in the direction of the secondary leg when pulses of Width III are being transmitted from the omni-directional antenna of the ground station. Thus, pulses of width I will be received at an aircraft only when the ground station directional antenna is looking at the aircraft, and the time interval between the reception at the aircraft of the previous pulse of width III and such pulses of width I will be a measure of the bearing of the aircraft relative to the secondary leg of the ground station. Therefore, the angle a (Fig 1) is also relayed to the aircraft.

Fig-2b is an expanded view of a portion of the wave form diagram of .Fig. 241, including one sequence scan of the ATC transmitted L-band pulses, and shows more clearly the relation between the ATO pulses of line A and the omnidirectional and directional pulses transmitted by a system according to this invention and represented by lines 13' and C respectively.

As will be apparent from Fig. 2b, it 'is required, according to the system, that a pulse of width I be sent up substantially simultaneously with the first pulse of each pulse pair of width V. If the first pulse of each pulse pair of width V were transmitted exactly simultaneously with a pulse of width I, it is possible that the two might, at certain bearings relative to the ground station, be received simultaneously by aircraft at those This would occur only infrequently, since pulses of width I are transmitted, directionally by a rotating antenna at the ground station, whereas pulses of width V are transmitted omni-directionally. However, when it did occur, the two pulses would be superimposed on the aircraft receiver, and the resultant wave form would depend upon the relationship between theradio frequency phases of the two signals. One solution to the problem is to arrange that each ground station transmit range information, by means of pulse pairs of width V, only on alternate one-second intervals, and disable the directional transmitter so that pulses of width I are not transmitted whenever a pulse of width V is being transmitted. With this arrangement, every other second is available for azimuth information alone. An objection to this method of operation is that it slows down the system by a factor of two, since navigation information is then corrected on the aircraft only every two seconds instead of every second. A better and preferred solution is to use short pulses for widths I and V and to arrange that pulses I and V are transmitted in sequence with a short interval at between them, as illustrated in Fig. 20. For example, pulse width I could be one microsecond in length and pulse width V two microseconds. The pulse of width I, as shown in line C", is transmitted simultaneously with the beginning of the corresponding ATC L-band pulse intended for the fourth altitude zone (line A"). The first pulse of a pair of pulses of width V (line B") would then be transmitted d microseconds after the pulse of width I. Thus, assuming d to be three microseconds, the pulse of width I and the first pulse of width V, together with the interval between them, require only six microseconds and, insofar as coincidence with the corresponding L-band ATC pulse is concerned, would be treated by the aircraft receiver like a single six-microsecond pulse from the ground station. When the aircraft is not' being swept by the rotating directional beam, the number I pulse is missing, but the first number V pulse follows the position of the potential number I pulse by three microseconds as before. While in Fig. 2c the ATC pulse in line A" has been indicated as being of greater duration than the combined widths of pulses I and V plus the intervening interval d, it is to be noted that the duration of this ATC pulse varies in accordance with the information as to adjacent traffic conditions transmitted by the ground station to aircraft in the particular altitude zone, which in the exemplified case is altitude zone 4. This does not raise any problem, however, since as will be pointed out hereinafter in connection with the block diagram of a system according to this invention, any S-band pulse received within ten microseconds after the leading edge of an L-band ATC pulse is treated as being received simultaneously with that L-band ATC pulse.

Fig. 3 is a block diagram of a ground station equipment according to the invention. Block 15 represents the ground station signal system of the cooperating ATC system and from this are fed L-band sync pulses once every 6 of a second to stabilize power source l6 in order to insure stabilization of the pulse transmission and of the rotation of the directional antenna. Stabilized power source I6 is fed from a separate main power source and its output is fed to a constant speed drive I! whose mechanical output at exactly 60 R. P. M. (1 R. P. S.) is coupled to commutators l8 and I9 and directional rotating antenna 20. L-band trigger pulses, both sync and altitude, are also fed from signal system l to the rotating slider 2| of commutator l8. Commutator l8 has ten segments 22 to 3| as shown. When rotating slider 2| is connected to commutator segment 22, pulses of Width III will be transmitted, denoting the secondary leg. As noted above, directional rotating antenna 20 is adjusted to make exactly one revolution per sec- 0nd and is synchronized so as to point in the direction of the secondary leg at the instant when slider 2| is connected to segment 22 and a pulse of width III is transmitted. When slider 2| continues in its rotation and connects with segments 23 and 24 in turn, pulses of widths II and IV respectively will be transmitted, denoting north and the primary leg respectively. When slider 2| rotates further and connects with segments 25 and 26 in turn, pulses of widths VI and VII will be transmitted, denoting the length of the secondary and primary legs respectively. Again, it is to be noted that the position of the secondary and primary legs and their respective lengths are determined for a particular ground station when the air lane is laid out with respect thereto, and hence the relative positions of commutator segments 22-26 are fixed at that time. The width of each of segments 22-26 is made just suflicient to insure the transmission of a single pulse of widths III, II, IV, VI, and VII respectively and each coincident with an L-band pulse, and thus each segment covers slightly less than one-third of a degree. The remaining segments 21-3!v of commutator l8 are connected together so that pulses of width I may be transmitted whenever pulses of widths II, III, IV, VI, and VII are not being transmitted.

To signal system l5 are coupled range tracking circuits 32 which determine the slant range to each aircraft from which L-band reply pulses are received. The output of range tracking circuits 32 is connected to horizontal range computers 33, which in known manner transfer each slant range indication into a horizontal range indication. The output of horizontal range computers 33 is connected to switching circuits 34, which are also controlled by triggers from signal system l5 so that each horizontal range is associated with the proper altitude zone and lane of the corresponding aircraft. The out ut of switchin circuits 34 is connected to a double pulse generator 35, which is controlled by triggers from signal system IS in a manner similar to that explained for switching circuits 34 and for the same reason. Double pulse generator 35 produces a first'trigger pulse coincident with the proper signal pulse from signal system l5 and a second pulse delayed thereafter by a time fixed by the output of horizontal range computers 33. The output of double pulse generator 35 is connected to the rotating slider 36 of the second commutator l9. Commutator I9 is similar to commutator l8 and has segments 31-4I corresponding to segments 21-31 of commutator l8. Insulatin segments 42-46 corresponding to segments 22-26 of commutator l8 are provided between segments 31- which are connected together and thence to modulator 41 to produce pulses of pulse width V from omni directional antenna 48 via omnidirectional transmitter 49. A delay circuit 50 is preferably inserted in the line leading from segments 37-41 to modulator 41 in order to delay the transmission of each pulse of width V the proper amount of time after the initiating trigger derived from commutator l9 in order to insure that a pulse of width V will never be received simultaneously with a pulse of width I at an aircraft, as explained above in connection with Fig. 2c. Segments 22-26 of commutator 3 are connected to other inputs of modulator 41 to produce omni-directional pulses of widths III, II, IV, VI and VII respectively as above-described. Segments 21-31 of commutator l8 are connected to modulator 5|; which in turn controls direcl1 tional transmitter 52 to produce pulses-of width 'l from rotating directional antenna 26.

Figs. 4a and 4b illustrate a block diagram of an airborne equipment for receiving the pulses transmitted according to the invention by the ground station equipment just described with reference to Fig. 3. The aircraft is assumed to be equipped also with an ATC system as described in the above application and hence includes an airborne L-band receiver 63 having an omnidirectional antenna 6l. The airborne equipment according to the invention includes. an S-band receiver 62 having an omni-directional antenna 63. Each S -band pulse will be received by omni-directional antenna 63 and thereafter conveyed through S-band receiver 62. In order to insure that S-band pulses of widths I-VII will be effective only if received from the nearest ground station, a gated stage '64.is provided after S-band receiver 62 and this is in turn controlled by a gate circuit 65 included in the ATC system and controlled by ATC L-band receiver 60. As explained in the above application, gate circuit 65 passes only the first L-band pulse of anytgroup received by the omni-directional. antenna 6|, thereafter failing to pass any pulse received by omni-directional antenna SI for approximately 926 microseconds. Since the first L-band pulse of a group received will necessarily be from the nearest ground station, thus only L-band pulses from the nearest ground station can pass gate .circuit 65 to trigger gated stage 64, which thereafter will pass any signal from S-band receiver 62 for a period of 10 microseconds after the trigger.

The output of gated stage6 l 'is connected to discriminators 66-42 in parallel. Discriminators 66-12 are each adjusted to pass only a pulse of a particular width of widthsI-VII respectively. Theoutput of discriminator 68, which passes only pulses of width III, is connected to timers -19 respectively, the connection to timers lil.9 being made through a sense switch 88 whose purpose will be explained more fully hereinafter. Each pulse of width III passed by discriminator 6 8 3 starts timers 15-19 operating. Thereafter, when an S-band pulse of width II from the nearest ground station is received at omni-directional antenna 63, it will be passed by receiver 62, gated stage 64 (since it is received simultaneously with a corresponding L-band pulse, either sync or altitude) and thence by discriminator 61, whose output is connected to timer T9 to stop its operation. One form which timers l5'l9 may take is illustrated in Figs. 5a-5c and will be explained more fully hereinafter. As pointed out above, and hence only briefly repeated here, each timer produces a mechanical angular output varying betweenzero and 360 and proportional to the time between the pulse starting the timer and the pulse stopping its operation.

When the subsequent omni-directional pulse of width IV from the nearest ground station is received by antenna 63, it is passed by receiver 62,

12 ondary and primarylegs respectively. Pulses of width I transmitted directionally from the nearest ground station are received by omni-directional antenna 63 whenever the ground station rotating directional antenna 20 (Fig. 3) is pointed at the aircraft, and these pulses are then passed by receiver '62, gated stage 64 and discriminator 66. These pulses of width'I passed by discriminator 65 are next fed to a circuit 8| which counts them and produces an output simultaneously with the median one. One form which this counter may take is shown in Fig. 6 and will be described more fully hereinafter. The purpose of this counter is, of course, to increase the accuracy of the azimuth indication of the aircraft relative to the ground station given to the aircraft by the ground station. It is necessary because even directional antennas have finite beam widths and hence the aircraft will receive several pulses of width I as the ground station directional antenna 20 (Fig. 3) rotates past it.

The output of counting circuit BI is fed through a gate circuit 82 which passes only one pulse per 0.1 second. This gate circuit 82 insures that only one pulse output from counting circuit 8! will be effective each timethe ground station directional antenna 28 (Fig. 3) rotates past the aircraft. The output of gate circuit 82 is fed to timer 18 to stop its operation, which has previously been started as above-described by the initial reception of a pulse of width III. The resulting mechanical angular output of timer i8 is equal to the bearing of the aircraft relative to the secondary leg of the ground station from which the pulses of width I are being received, or in other words, angle a (Fig. 1).

The first pulse of each pair of pulses of Width V received at S-band receiver 62 via omni-directional antenna 63 is similarly passed by gated stage 64 and discriminator 18. The output of discriminator 10 is fed to altitude and lane gate circuit 85, to which is also fed a corresponding trigger from the ATC system altimeter switch I to insure that the pulse of Width V is intended for the particular altitude zone lane in which the aircraft is located. The output of altitude and lane gate circuit 85 is then fed to gate circuit 86 and range tracking circuit 81 in parallel. A second discriminator 88, which passes only pulses of width V, is connected between the output of S-band receiver 62 and gate circuit 86. Gate circuit 86 is triggered on from the output of discriminator T6 for an interval equal to microseconds thereafter, which is the maximum allowable time interval between the individual pulses of each pulse pair of width V. The output of gatecircuit 86 is connected to range tracking circuit 81 also. Thus, the first pulse of a pulse pair of width V received at the aircraft is passed by gated stage 64, discriminator l8, and altitude and lane gate circuit 85 to open gate circuit 86 and also to start the operation of range tracking circuit 81. This first pulse is also passed by discriminator 68, but does not pass gate circuit 86 because the latter has not been triggered on yet. The second pulse. of the pulse pair of width V is not passed by gated stage 64 because it is not received within ten'microseconds after the corresponding L-band ATC pulse, but is passed by discriminator 88 and gate circuit 86 to stop the operation of range tracking circuit 81, whose output is a voltage proportional to the time interval between the pulses of the pulse pair and thus proportional to the horizontal range of the 13 aircraft from the ground station as .determined at the ground station.

The mechanical angular outputs of timers I and I6 are connected to linear potentiometers 99 and 9| respectively, whose outputs are thus voltages proportional respectively to the length of the secondary and primary legs of the nearest ground station. The mechanical angular outputs of timers II I9 are connected to resolvers 9294 respectively. Each resolver is fed with a trapezoidal voltage input derived from trapezoidal generator 95. Generator 95 in turn derives its input from multivibrator 96, which is triggered on by master trigger pulses (both sync and altitude) from the airborne ATC signal system. Resolver 94 is fed with a second mechanical angular input equal to the heading of the aircraft relative to north from a gyrosyn repeater 91, which in turn is controlled by the fluxgate compass of the aircraft. The voltage outputs of resolver 92 will thus be a measure of the bearing of the primary leg of the air lane relative to the secondary leg, the voltage outputs of resolver 93 will be a measure of the bearing of the aircraft relative to the secondary leg of the air lane, and the voltage outputs of resolver 94 will be a measure of the heading of the aircraft relative to the secondary leg.

The mechanical input for each of timers I5I9 is derived from motor I39, whose power is supplied from a 60-cycle source and is stabilized by the ATC sync trigger pulses in a manner similar to that above-described in connection with the constant speed drive I! of Fig. 3. Motor I39 rotates at exactly 60 R. P. M. (1 R. P. 8.). Another motor 98 is mechanically connected to the sliders I99-I 94 of five commutators I96I I9 respectively, each of which has four segments electrically insulated from one another and each covering substantially 90 degrees. This motor 98 operates at 1800 R. P. M. The four output leads from each of resolvers 92-94 are connected to corresponding segments of commutators I9III9. Sliders I9I and I92 of commutators I91 and I98 respectively are connected to the inputs of vertical drivers and clampers III, whose output is connected to vertical deflecting elements I I2 of cathode ray tube indicator I. Similarly, sliders I93 and I94 of commutators I99 and H9 respectively are connected to the inputs of horizontal drivers and clampers I I3, whose output is connected to horizontal deflecting elements H4 of cathode ray tube indicator I. The fourth segment of commutators I98-I I9 is connected to ground through a biasing source II5 of sufilcient magnitude to cut off horizontal drivers and clampers I I3 and that vertical driver of block II I which produces the upward vertical sweep. The corresponding fourth segment of commutator I9! is connected to the output of trapezoidal generator 95 and provides energy for that vertical driver of block III which produces the downward vertical sweep. Slider arm I99 of commutator I96 is connected to the input of intensifier amplifier H6, whose output is connected to beam intensifier element II! of cathode ray tube indicator I. The output of linear potentiometer 99 is connected to a multivibrator gate generator I29, to which is also fed master trigger signals from the ATC signal system, and its Output is connected to one segment of commutator I 96. The output of linear potentiometer 9| is connected to multivibrator gate generator I2 I, which similarly is fed with triggers from the ATC signal system to initiate its operation, and

its output is connected to a second segment of commutator I96. The voltage output of range tracking circuit 8! is connected to phantastron and diiferentiator I22, which similarly has its operation initiated by master trigger pulses from the ATC signal system, and its output is connected to a third segment of commutator I 96. The fourth segment of commutator I96 is fed from gate generator I23, which in turn is controlled by delay multivibrator I24, whose input is connected to the source of master trigger pulses from the ATC signal system.

The purpose of commutators I9III9 is to feed successively to cathode ray tube indicator I sweep voltages at angular positions corresponding to the relative positions of the secondary leg,

the aircraft relative to the secondary leg, the

heading of the aircraft relative to the secondary leg, and the primary leg relative to the secondary leg. The purpose of commutator I96 is to properly intensify the beam of the cathode ray tube indicator I at these angular positions. As indicated, each commutator has only four segments, and thus a trace on the cathode ray tube indicator I is produced for one one-hundred-twentieth of a second each one-thirtieth second, the four desired indicia being displayed in turn, and persistence of luminescence of the indicator screen as well as persistence of vision of the observer being relied upon to insure a continuous visual presentation. Note that since the sweep circuits are triggered on by the master pulses from the ATC signal system, which occur at the rate of 1080 per second, each trace is repeated approximately or 9 times duringeach one one-hundred-twen tieth second that each slider arm of a commutator is connected to a single segment. It is, of course, obvious that the number of segments can be increased as desired to include additional indicia, such as, for instance, the position of neighboring aircraft within the same block. Also the number of segments can be doubled, tripled, etc., if necessary to prevent flicker of the presentation on the cathode ray tube indicator face.

As pointed out above, reception of each pulse of width III by omni-directional antenna 63 simultaneous with an L-band ATC pulse by omnidirectional antenna BI of the ATC system starts the operation of each of timers IE-l9. Each of these timers has its operation stopped by the receipt via omni-directional antenna 63 of the proper subsequent S-band pulse simultaneous with the reception of a corresponding L-band ATC pulse via omni-directional antenna SI of the ATC system. Thus, receipt of a subsequent pulse of width VI stops the operation of timer I5 such that its mechanical angular output is a measure, as a fraction of 360 degrees, of the length of the secondary leg of the air lane. In the same fashion, pulses of widths VII, IV, I and II stop the operations of timers l6i9 respectively to produce respective mechanical angular outputs denoting the length of the primary leg, the angular position of the primary leg relative to the secondary leg, the angular position of the aircraft relative to the secondary leg, and north relative to the secondary leg respectively, each as a function of 360 degrees. .By applying the mechanical an-' gular outputs of timers I5 and I6 to linear potentiometers 99 and 9| and supplying each of these. potentiometers with. a suitable voltage, the

resultant voltage outputsfrom their sliders, relative to the total voltage" applied is a measure of the lengths of the secondary and primary legs of the air lane respectively. These voltages from the sliders are then applied to multivibrator gate generators I20 and I2.I respectively; Whoseoperation is initiated every /1080 ofa second by master trigger pulses, both sync and. altitude, from the ATC signal system- Each multivibrator gate generator I20 and I2I thus produces a gate output starting simultaneously with each L-band ATC pulse received by the airborne equipment, the ending of the voltage gate output of these generators I20 and I2I being determined by the voltage output from linear potentiometers and 0i respectively. These voltage gates are then applied by meansof commutator I06 for the proper length of time to fix the length of the.

corresponding radial traces on the cathode ray tube indicator I proportional to the lengths of the secondary and primary legs respectively. ofthe air lane at the nearest ground station. The angular position of these radial air lane le traces is determined by means of resolver 92 and biasing source I I 5. Since the deflecting elements of cathode ray tube indicator I exemplified are in the form of electromagnetic coils, it is, of course, necessary to provide trapezoidal sweep voltages, which are derived from trapezoidal generator 05. Note that the segments of commutators I00-IIO are so oriented that the segment of each corresponding to each of the four indicia to .be displayed'on the face of the cathode ray tube indicator I is traversed at the same time and for the same period of time. Thus, whenever slider I00 of commutator I06 is connected to its segment corresponding to the secondary leg, fed from multivibrator gate generator I20, slider IIH of commutator I01 is also connected to its corresponding segment, fed by trapezoidal generator 05, and sliders I02-I04 of commutators I08I I0 are each connected to their corresponding segment, which is fed by biasing source II5. Thus, during the one one-hundred-twentieth second of each one-thirtieth second during which sliders I00-I04 are connected to these segments of commutators I06I I0, a vertical radial trace extending downwardly from the center on the face of the cathode ray tube indicator I is produced, its length being fixed by the mechanical angular output of timer I as above-described. Resolvers 02 and 93 resolve the output from trapezoidal generator 95 into the proper sine and co-sine components as determined by the mechanical angular output of their respective timers TI and I8, which sine and co-sine components are then fed to vertical drivers and clampers III and horizontal drivers and clampers H3 at and for the times fixed by commutators I0'II I0 to produce the proper angular positions of the traces corresponding respectively to the positions of the primary leg and the aircraft relative to the secondary leg, which, as has been seen above, is arbitrarily fixed on the face of the cathode ray tube indicator I so as to be always in a vertical downwardly extending position. The beam of the cathode ray tube at the angular position corresponding to that of the aircraft is intensified only at a radial distance proportional .to the horizontal range of. the aircraft from the ground station at C (Fig. 1) this being obtained'by means of the differentiated output of the phanta'stron included in block I22.

Similarly, the beam produced at the angular position corresponding tothe heading of the air-- craft is intensified only for a relatively short length adjacent the periphery of the face. of. the cathode ray tube indicator I, the length of this trace being determined via slider I00 of commutator I06 by the duration of the voltage gate output ofgate generator I23 and its start being fixed by the output of delay multivibrator I24. The angular position of the heading of the aircraft is derived from resolver 04 by means of the fourth segment of each of commutators IOT-I I0. Note that if the mechanical output of gyrosyn repeater 91 were not also fed into resolver 94, the electrical output from the latter would produce a radial traceon cathode ray tube indicator I denoting north relative to the secondary leg. Since the heading of the aircraft is of more use to the pilot, the additional mechanical output from gyrosyn repeater 91 is provided which in known manner varies the resolver voltage output to give the'heading of the aircraft instead of north relative to the secondary leg.

Sense switch 80. is included because the secondary leg of the air lane is arbitrarily defined as the leg of a particular block which an aircraft traverses in flying toward the block station of that block and, as noted above, the secondary leg has been arbitrarily fixed so as always to be in a vertical downwardly extending position on the face of cathode ray tube indicator I. In the position of sense switch 80 shown, it has been assumed that the aircraft exemplified is flying in a generally northerly rather than southerly position, and that the air lane runs generally north and south. On its return flight, the aircraft pilot would operate sense switch 80 so that the connections from discriminators II. and I2, and 68 and 69 would be reversed, which will result in the proper orientation of the four indicia (primary and secondary legs, aircraft position, and aircraft heading) on the face of cathode ray tube indicator I for a south-bound aircraft. While the use of sense switch 80 has been explained for northerly or southerly flights along a generally north-south air lane, it will be obvious that the same holds true for easterly or westerly flights along a generally eastwest air lane.

InFigs. 5a-5c is illustrated diagrammatically one form of an electromechanical timer which may be utilized in the equipment of Figs. la-4b. Each timer has two similar halves as shown in Fig. 5a, which operate alternately to produce a mechanical angular output, the magnitude of this output being determined by the spacing between two subsequent pulses applied to the control circuit thereof, which is illustrated in Fig. 50. Each half includes a fixed magnetic yoke I30 which is fabricated in the form of a cylindrical can or cup having a central internal shaft. Each half thus has a generally E-shaped cross section as shown in Fig. 5a. To the free end of each central shaft is afiixed a gear I3], and three windings I32--I34 are provided. on the yoke as shown. Adjacent each gear I3I and coaxial therewith is provided a second gear I35, of the same diameter and having the same number of teeth and driven from motor I30 (Fig. 4b) at exactly 1 R. P. S. Between each gear I3I and I35 is pivotally secured a lever I36, the pivot axis being coaxial with the center of gears I3I and I35. A transverse arm I3! is pivotally secured to lever: I36 adjacent the periphery of gears I3I and" I35 as shown. The end of arm I31 adjacent gear I35 is provided with an ear I38 adapted to. beam 17 gagedv between any two adjacent teeth of gear I35 when arm I31 is rotated slightly clockwise. The arm I3! is a permanent magnet with a north pole N adjacent to gear I3I. The end of magnet I37 adjacent gear I3I is similarly fashioned so as to be engaged between any two adjacent teeth of gear I3I when arm I3'I is rotated slightly counter-clockwise. A yoke I40 is pivotally secured coaxially with gears I3! and I35 and the pivot points of levers I36, and is provided at one end with an output shaft I4I as well as a zero positioning spring I42. Levers I36 and I35 are of sumcient length to engage yoke I46 when either lever pivots about its pivot axis. Springs I43 may also be provided between lever I36 and arm I31 to maintain these parts normally at right angles to one another so that in the absence of a voltage applied to coils I 32-434, ear I38 does not engage gear I35 and magnet I3? does not engage gear I3I. Also, a zero positioning spring I44 may be provided for each lever I36.

Looking at Fig. 5a from the left, as in Fig. 5b, motor I39 is assumed to be rotating in a counterclockwise direction. Directing attention first to the left-hand half of Fig. 5a, coil I32 is so Wound that when it is energized the periphery -of gear I 3| will assume a north magnetic polarity so that the adjacent end of magnet I 31, which is also of north polarity as shown, will be repelled. Arm I 3'! will, therefore, be rotated clockwise until ear I38 is engaged between two of the teeth of gear I35. Lever I36. will therefore move in a counter-clockwisedirection (Fig. 5b) simultaneous with the movement of gear I35 at l R. P. S. and will carry yoke I40 along with it until ear I38 is disengaged from gear I35. The disengagement of ear I38 from gear I35 is accomplished by means of coil I34, which reverses the polarity of the periphery of gear I3I so that the end of magnet I 31 adjacent gear I3I is attracted to the latter and held between two of its gear teeth. This is accomplished by energizing coil I34 to a sufiicient degree to overcome the magnetism produced by coil I32, which is then deenergized. Yoke I40 is thus held at an angular position determined by the time interval elapsing between the energization of coils I32 and I34. As will be discussed hereinafter in detail in connection with Fig. 5c, the right-hand half of Fig. 5a, is next operated in the same fashion as that just described for the left-hand half. Lever I36 is moved from its zero position, from which it always starts due to the tension of spring I44 when start coil I32 is energized and ear I33 engages gear I35 tion at which lever I36 stops is determined by the time thereafter at which stop coil I34 is energized, start coil I32 again being deenergized at the same instant. If the angular position assumed by lever I36 is greater than that initially assumed by lever I36, yoke I40 will be carried on to the new position assumed by lever I36. On the other hand, if the angular position assumed by-lever I36 is less than that previously assumed by lever I36, yoke I46 will thereafter be drawn back into engagement at the angular position of lever I36 due to the urging of spring I42, because at the same instant that stop coil I34 is energized all three coils (I32-I34) of the lefthand half of Fig. 50 will be de-energized so that arm I31 will return to its normal position perpendicular to lever I36, and lever I36 will thus return to its zeroposition due to the urging of spring I44. The cycle above described is then repeated indefinitely as long as coils I32, I34, I32 and The resultant angular posi- 18 I34 are energized in succession, levers I36 and I36 determining the angular position of yoke I alternately, and hence the angular position of output shaft I4I. Coils I32-I34 of the righthand half of Fig. 5a are, of course, also deenergized at the same instant that stop coil I34 of the left-hand half is energized, thus permitting the alternate operation of yoke I40 by levers I36 and I36 as described. Coils I33 and I33 are hold coils, whose operation will be explained fully hereinafter in connection with the detailed explanation with regard to Fig. 50.

Note in Figs. 5a and 512 that a limit switch I45, having contacts connected to leads 2! and 22, is provided at the top of the mechanism, which is the normal or zero position of yoke I40, and is so positioned that the control circuit for the timer, to which it is connected by means of leads 2i and a2, will be disabled should yoke I40 ever attempt to rotate more than its theoretical limit of 360 degrees. Actually, this maximum figure will be nearer 357 degrees, since at least three degrees will be required for a rigid mounting to support the internal parts of the timer, such as the motor. Such a support may be provided in the form of a sheet of metal extending from an external support to the internal parts, this sheet of metal being located within a three-degree are through which the yoke I40 does not rotate. Thus, limit switch I actually will be adjusted to stop the operation of the timer should yoke I40 attempt to rotate more than 35'7 degrees. A block of insulation I46 is provided on yoke I40 and positioned properly to engage the movable contact of limit switch I45, and a spring I41 is prcvided to maintain this movable contact in engagement with its cooperating fixed contact except when also engaged by insulation block I46. A limit switch I48 is also provided adjacent the zero position of each lever I36, this limit switch I48 being shown in Figs. 5a and 5b adjacent magnet I31. The contacts of each limit switch I43 are normally'held open by means of a spring I49, as shown in Fig. 5c, but are held in engagement with one another whenever the corresponding lever I36 or I36 is in its zero position, as will be pointed out hereinafter in connection with the discussion of the operation of the control system of the timer shown in Fig. 50. These zero limit switches I46 and I48 insure that at least onev of the levers I36 and I36 is in its zero position before operation of the timer can be initiated, and thus insure correct operation.

Referring now to Fig. 5c, the start terminal of the timer is connected through the contacts of limit switches I48 and I48 in parallel to one terminal of coupling. condenser I50. The other terminal of coupling condenser I50 is connected to the control grid of screen grid thyratron I5I. The control grid and cathode of this thyratron IEI are also coupled to ground through biasing resistors in a conventional manner as shown. The screen grid of this thyratron I5I is connected. to a suitable source of positive potential through a resistor I52, and its plate is connected to the same source of positive potential through limit switch I45 and either start coil I32 or I32 depending upon the position of armature I53, whose position is determined by relay I54. The stop terminal of the timer is coupled to the control grid of a second screen grid thyratron I through coupling condenser I56 The control grid of thyratron I55 is also coupled to ground by a biasing resistor I51, its cathode is directly connected to ground, and its screen grid is connected to a negative source of potential through resistor I58 and to the screen grid of thyratron II through resistor I59. This screen grid connection between thyratrons I5I and I55 insures that stop tube I55 cannot fire if start tube I5I has not previously fired. The plate of stop thyratron I55 is connected to a suitable source of positive potential through armature I60 associated with relay I54 and alternatively either stop coil I34 and one of the coils of relay I54 or stop coil I34 and the other coil of relay I54. Hold coils I33 and I33 are alternately con-'- nected as shown in circuit with a source of potential I6I by means of contact I62 associated with relay I54.

The operation of the control circuit of the timer is as follows. Any pulse passed by the associated discriminator 68 (Fig. 4a) thereafter appears at the start terminal of the timer and will be coupled via coupling condenser I50 and either limit switch I48 or I48 (or both in parallel if the equipment is just being started) to trigger on start thyratron I5I, assuming that yoke I40 Figs. 561-517) is not jammed against limit switch I45 and that a positive potential is thus applied to the plate of this thyratron I5I through either start coil I32 or I32 depending upon the position of armature I53. When start thyratron I5I is triggered on by this initial pulse, it fires, drawing current through either start coil I32 or I32, whichever is connected in the plate circuit as above-described. For the purpose of this explanation, it will be assumed that armature I53 is in the position shown and thus current is drawn through start coil I32. Therefore, as explained above in connection with Figs. 5a-5b, ear I38 engages gear I35 and, assuming that motor I39 is rotating at its-fixed speed of l R. P. S., lever I36 starts revolving. Thereafter, when a subsequent pulse is applied to the stop terminal of the timer from either gate circuit 82, discriminator 61, discriminator 69, discriminator H, or discriminator 12 (Fig. 411), this pulse is coupled to the grid of stop thyratron I55 via coupling condenser I56. Since start thyratron I5I has already been fired and is drawing current through resistor I52, the bias potential on the screen grid of stop thyratron I55 is reduced sufficiently due to resistor network I52, I58, and I59 shown so that the negative bias normally applied thereto by means of resistor I58 is overcome and stop thyratron I55 also fires. Since its plate is coupled to a suitable source of potential through armature I60, stop coil I34, and one of the coils of relay I54, the firing of stop thyratron I55 causes current to pass through stop coil I34 which, as explained previously in connection with Fig. 5a., withdraws ear I38 from engagement with the teeth of gear I35 and causes the opposite end of the permanent magnet arm I31 to engage with the teeth of gear I3I. This same current flows through the left-hand coil of relay I54 which, as shown by its arrow, causes armatures I53, I60 and I62 to move to the right, thus de-energizing start coil I32 and stop coil I34 and energizing hold coil I33. The contacts associated with armatures I60 and I62 are so arranged that hold coils I33 and I33 are always energized before movement of armature I60 de-energizes the corresponding stop coils I34 or I34. By choosing thyratrons I5I and I55 having a sufiiciently short de-ionization time, each of these tubes ceases firing during the switching of relay armatures I53, I60 and I62 so that while current still flows 20 I through hold coil I 33, both thyratrons I5I and I55 are off and hence no current flows through start coil I32 or stop coil I34 when the switching of these relay armatures is completed. Thereafter, when a subsequent pulse is applied from discriminator 68 (Fig. 4a) to start thyratron I5I, current flows through start coil I32 and lever I36 is moved to an angular position, as described above in connection with Fig. 5a, whose magnitude is determined by the interval elapsing until the reception of a subsequent pulse by stop thyratron I55, thus causing a current to flow through stop coil I34. This same current flows through the right-hand coil of relay I54 which, as indicated by its arrow, causes armatures I53, I60 and I52 to move back to the position shown, de-energizing start coil I32 and stop coil I34 and energizing hold coil I33 prior thereto. Again, both thyratrons I5I and I55, due to their short de-ionization time, cease firing and the cycle is then ready to begin anew upon the reception of subsequent pulses by the timer.

Note that for accurate operation of the timer its driving source, motor I39, must run at exactly the same speed as the corresponding commutators I8 and I9 and antenna 20 (Fig. 3) of the ground system. This is insured by designing both the ground station motor and the airborne motor to operate at exactly the same speed (1 R. P. S.) and supplying each of these motors from a stabilized power source, preferably stabilized by synchronized means. In the exemplified case this synchronized means is, of course, the L-band ATC pulses.

In Fig. 6 is shown one form of couting circuit which may be utilized in block 8| of Fig. 4a. As explained above, such a counting circuit is required because even directional antennas have finite beam widths, and. hence several pulses of width I are received at the aircraft as the ground station directional rotating antenna 20 (Fig. 3) rotates past the aircraft. For precise measurement of the angle of the aircraft relative to the ground station, it is obvious that only the median one of these pulses received should be effective to operate the indicator of the navigation system, and this is the reason for the inclusion of counting circuit 8|. All pulses of width I passed by discriminator 66 (Fig. 4a) are applied as shown to the control grid of trigger tube I65. These pulses cause trigger tube I65 to fire blocking oscillator I66, which comprises delay line I61, pulse transformer I68, and tube I 69. The output of blocking oscillator I66 is fed into a step-by-stepcounter circuit I10, of which diodes HI and I12 are a part and connected as shown. Thus, there is a voltage established across the condenser I13, connected between the cathode of diode I12 and ground, Which is proportional to the number of pulses of width I received per second. The purpose of blocking oscillator I66 is to provide uniform pulses to the counter circuit I10 regardless of the nature of the pulses received at S-band receiver 62 (Fig. 4a). The voltage across condenser I13 is connected to the grid of tube I14 to bias this tube and thus control the amount of delay introduced by the cathode-coupled multivibrator I15, comprising tubes I14 and I16. The output of pulse width I discriminator 66 (Fig. 4a) is also connected to the grid of tube I14, and thus cathodecoupled multivibrator I15 is triggered by the first pulse of the group of pulses of width I which are received each time the rotating beam from the ground station rotating directional antenna 20 (Fig. 3) sweeps past the aircraft. The output of The second pulse is negative. and is passed by diode I88 to produce a positive output across its plate resistor I8I By properly proportioning the circuit parameters the output at the plate of diode I88, which is then connected .to gate circuit; 82 (Fig. 4a), will be delayed by a time interval equal to one-half the time required for the complete beam to sweep by the aircraft. Thus the desired result is obtained.

A more precise circuit can be made by replacing multivibrator I75. with a phantastron.

InFig. 7 is shown one form of range tracking circuit which maybe utilized in block 81 of 4a. This circuit is one form of the so-called regenerative tracking circuit, and its purpose is to convert the time interval between two pulses representing the range of the aircraft from the ground station into a voltage which is proportional to the range. This conversion is subject to the requirement that it shall not be disturbed by the presence of a third. pulse. The first pulse of each pair of range pulses of width V appears at the output of altitude and lane gate circuit85 of Fig. 4a and is coupled by means of condenser I85 to phantastron tube I86 as shown. This pulse triggers the phantastron 'to produce an output cr s athod e sto l 81 ofwthqde oll we tube I88. Diode I 89 clamps theplate oi tube I86 to the voltage appearingatthe. upper end of resistor I 98 and establishes the level from which the downward excursion of the plate voltage of tube I86 begins, thus controlling the duration of the phantastron switching action. "li'he output of the cathode follower I88 is shaped by means of the differentiating circuit consisting of condenser I 9! and resistor I92. The positive swing produced at the end of the phantastron switching action, after difierentiation, triggers the succeeding blocking oscillator tube I93. The output of the blocking oscillator is limited to a positive pulse by diode I94, and this output appears across its cathode resistor I95. The purpose of blocking oscillator I93 is to sharpen the output of the phantastron. The purpose of the stages represented by tube I 86, I88, I93, and I94 is to produce a pulse across resistor I95 which is delayed with respect to the original pulse at condenser I85 by a time interval proportional to the voltage across resistor I98,

The pulse appearing across resistor I95 is applied simultaneously to a delay line I96 and to the grid of triode I91, which with triode I98 forms a flip-over circuit. This pulse cuts off tube I98, which is normally on, orconducting. When a negative pulse returns-after reflection, from the far end of the delayline I96, tube I9! is cut ofi and tube I98 is restored to a conducting condition. There is thus generated at the plate of tube I98.a gating voltage, the due ration of which is equal to the time required for a pulse to travel down the delay line I95 and return. For illustrative purposes, this time will be taken as 25 micro-seconds. The gating voltage produced at the plate oftube I98 is pplied to the screen grid of pentode I99. The output of gate circuit 86 (Fig. 4a) is coupled to the control grid of pentode I99 by means of condenser 288. If the delaytime is correct, pentode 4 w l ass h second. Puls Q? the ir ar a e I and returns to its quiescent state.

the screen grids of pentodes 282. and 203m parallel. The control grids of pentodes 28 2 and 283 are biased so that when single-shot multivibrator 284 composed of tubes 265 and 286 is in its quiescent state with tube 286conducting, pentode 283p produces a negative pulse at its. plate and pentode; 282 produces no output when the above-men tioned positive pulse from tube 28I is applied to their screen grids The far end of delay line I96 is connected to thegrid of tube 285 through condenser 281 as shown. Thus, each pulse applied to the near end of the delay line I96 (which is connected to the grid of tube I97) arrives at the far end of delay line I96 at a time repre-v senting the mid-point of the 25 microsecond gate (12 /2 microseconds) and is applied as a trigger to the grid of tube 285. Tube 286 is then out oh and tube 285 begins conducting. After a time greater than the-length of the gate generated by tube I98, single-shot multivibrator 284 recovers The negative gate generated at the plate of tube 285 is applied to the control grid of tube 283 and makes it incapable of producing an output While tube 285 is conducting. Similarly, the positive gate produced at the plate of tube 286 is applied to the control grid of tube 282 and makes it capable of producing a negative pulse at its plate in re-v sponse to a positive pulse on its screen gridduring the interval during which tube 288 is out oii and tube 285 is conducting. Each negative output pulse produced by tube 282 charges c0 ndenser 288 by the action of diode 289 and raises the potential of the grid of cathode follower 218, whose cathode is connected to ground through resistor I99, Each negative output pulse produced by tube 283 removes charge from cone denser 288 by the action of diode 2i I and reduces the potential of the grid of cathode follower 2I8. Thus, if the second pulse of the pulse pair of width V arrives via gate circuit 86 (Fig. 4a) before the delayed pulse triggers the single-shot multivibrator 284, i. e., during the first half of the 2 5 microsecond gate, tube 283 produces an output, and the voltage on the grid of cathode follower 2I8. and hence across its cathode resistor I98 is decreased. Decrease in the voltage across resistor I98 decreases the delay of phantastron I86 and causes the 25 microsecond gate to occur earlier. If, on the other hand, the second pulse of the pulse pair of width V arrives after the delayed pulse triggers the single-shot multivibrator 284, i. e., during the second half of the, 25 microsecond gate, tube 282 produces an output, and the potential on the grid of cathode follower 2I8 and hence across its cathode resistor I98 is increased. The increased voltage across resistor I98 increases the delay of phantastron I86 and causes the 25.microsecond pulse to occur later. The net effect is to center the output of gate circuit 86, in the 26 microsecond gate. Since, the delay in the phantastron circuit is proportional to the volt-age across resistor I98, and since-this delay equals the. spacing between the pulses from the gate circuits. '85 and 86 (Fig la) [and hence from the range discriminators I8, and 88 of Fig. 4a], the voltage across resistor I98 is proportional to the spacing between the range pulses. This voltage, which, as explained, .is variable in accordance with the spacing between the range pulses, then connected to the plate 

